Test structures are often utilized in the semiconductor device industry as a method for rapidly developing a process flow. Many existing methodologies currently exist such as KLA Tencor's uLoop™, Applied Material's, Inc. ADL™ and the Real Vision™ jointly developed by Applied Materials Inc. and PDF Solutions Inc. These techniques allow a large number of devices to be rapidly tested allowing determination of root cause failure analysis in a relatively short period of time.
There are two basic requirements of any such technique to be useful for rapid diagnostics and root cause failure analysis. These are 1) detection of the electrical error and 2) localization of the electrical error for failure analysis. For example, electrical test by probe techniques can rapidly measure a very large number of devices. However, when an electrical error is detected it is only localized to a somewhat large area on the test chip. This means that a failure technique such as focused ion beam cross sectioning can only be applied if the defect is further localized by a following technique such as ADL™.
Common to most techniques such as those mentioned above is a relation between the sensitivity to resistance failures, and the amount of current that can be driven into the device. This is in general true because the voltage that can be generated across a resistive error is determined by the current as dictated by Ohm's law. This is particularly important for electron microscopy techniques where the current density determines the amount of current that can be injected into a small feature. The total current in the illuminating beam can be made arbitrarily large, but only by allowing the spot size to also become large.
The invention herein is specifically directed towards electron microscopy techniques and as such the following background material, which pertains specifically to electron microscopy, is presented. As the device scale shrinks to 45 nm technology and beyond, the amount of current that can be injected into an individual device feature becomes limited. For example, using conventional retarding field electron optics, it is possible to provide 200 nanoamperes into a Gaussian shaped probe with a FWHM of 100 nm. A typical feature such as a contact will have a nominal dimension of 1.5 times the device node so that a contact of the 45 nm nose would have a diameter of approximately 70 nm. In this case, only about 30% of the primary beam current or 60 nA would strike the contact. The other 70% would only contribute to the noise of the measurement.
When testing electrically contiguous structures of large spatial extent, the capacitance of the structure will grow proportionally with the area subtended. The total dose required to saturate the capacitance and reach the resistive regime of the electrical response of the structure is therefore also proportional to the area subtended by the structure. The total dose is the product of the current injected by the probe and the time that the probe impinges upon the structure. Typically, as in the case of via chain and metal comb test structures, the defect of interest is a faulty contact or metal short respectively. In both cases, these are resistive defects and the capacitance of the structure is not of interest. In these cases, if the exposure time is short relative to the RC response of the structure with capacitance C containing a defect with capacitance R, the sensitivity will be compromised. To maximize sensitivity, it is necessary to expose the structure under test for a sufficient amount of time to saturate the capacitance of the structure. Accordingly, improved test structures are desirable.